Dysregulated transcription is frequently involved in cancer development. Chromatin immunoprecipitation (ChIP) assay is the technique of choice for examining in vivo transcription factor-DNA interactions. The current ChIP technology is limited by the requirement of a large number of cells and the long assay time caused by extensive manual processing. These problems practically prevent its use on primary cells extracted from animals and patients. In this project, we will develop microfluidic ChIP assays for studies based on tiny amounts of primary samples from mice and humans. We will achieve a sensitivity of ~20-50 cells for ChIP-qPCR and ~1000 cells for ChIP-seq for transcription factor binding studies using primary cells by the end of this project. Taking advantage of the ultrahigh sensitivity, we will demonstrate two innovative applications of ChIP assays: 1. Study transcriptional regulation in a cellular subset from primary tumors, namely tumor-initiating cells (TICs); 2. Monitor temporal dynamics in transcriptional regulation by minimally-invasive examination of a single live mouse. These experiments cannot be conducted using current technology and will yield unique insights that improve the understanding of cancer development at the molecular level.